[lit] Add HOME to "safe" variables to pass through to tests (#139367)

Incldue `HOME` among the variables that are passed through to tests when
environment is cleaned. This is necessary for Gentoo build environments,
where individual package builds are given temporary home directories
that are exposed via `HOME` variable. By stripping the variable, `lit`
made these tests attempt to access user's home directory, resulting in
permission errors.

Author: mgorny

lit/TestingConfig.py

@@ -66,6 +66,7 @@ def fromdefaults(litConfig):
             "DFLTCC",
             "QEMU_LD_PREFIX",
             "QEMU_CPU",
+            "HOME",
         ]
 
         if sys.platform.startswith("aix"):